DETAILED DESCRIPTION OF THE INVENTION
1. Technological Field of the Invention
The present application is a U.S. non-provisional application based upon and claiming priority from Japanese Application No. HEI 10-248690 which is hereby incorporated by reference.
The present invention relates to a method for manufacturing a high-quality optical polycarbonate that has a superior hue and very few microparticles formed from inorganic or organic foreign matter or the like.
2. Technological Background of the Invention
Because of their excellent impact resistance and other mechanical properties, and their excellent heat resistance, transparency, and so on, polycarbonates are widely used in applications such as various mechanical parts, optical disks, and automotive parts. They are hold particular promise for optical applications, such as memory-use optical disks, optical fibers, and lenses, and research is underway on a number of fronts.
Known methods for manufacturing these polycarbonates include a method in which a bisphenol such as bisphenol A is allowed to react directly with phosgene (interfacial method), and a method in which a bisphenol such as bisphenol A and a carbonic diester such as diphenyl carbonate are subjected to a melt polycondensation reaction (ester interchange reaction).
The interfacial methods involving phosgene that are commonly employed today require the use of a large amount of solvent such as methylene chloride, so it is extremely difficult to remove the chlorine, making these products undesirable as optical polycarbonates.
Meanwhile, an advantage of a melt polycondensation reaction method is that a polycarbonate can be manufactured less expensively than with an interfacial method. Also, no phosgene or other toxic substance is used, and no methylene chloride or other such solvent is necessary, so this method is very promising for the manufacture of optical polycarbonates.
A polycarbonate produced in a commercial plant generally contains between 5000 and 100,000 microparticles of submicron size per gram, and sometimes more than 10,000 microparticles are contained. There is also a great deal of variance in the number of microparticles contained in these polycarbonates.
When a polycarbonate is used in an optical application such as an optical disk, any microparticles on the order of microns that are admixed in the polycarbonate will become a source of light scattering, so the optical performance will be inconsistent and noise will be generated. Accordingly, a polycarbonate used in an optical disk needs to have foreign matter removed from it at a particularly high level of precision. The most common way to remove these is by using a filter. With a melt polycondensation reaction method, however, the obtained polycarbonate has a high melt viscosity, and when high precision filtration is performed using a filter directly, the differential pressure of the filter and other such factors impose a limit [on the filtration precision], making it very difficult to remove microparticles of submicron size. Meanwhile, there are methods for reducing the quantity of microparticles in a polycarbonate by removing the microparticles from the bisphenol used as a raw material in the manufacture of the polycarbonate. For instance, Japanese Laid-Open Patent Application 08-325184 discloses that excellent results will be obtained in terms of chemical stability and filtration precision if a membrane filter made of a fluororesin is used in the filtration of a bisphenol.
However, a problem encountered in filtering a molten bisphenol using a membrane filter made of a fluororesin is that the bisphenol has such a high surface tension that it does not readily pass through the filter. A complicated procedure is therefore required in which the surface of the fluororesin membrane filter is first wetted with a liquid having a lower surface tension, and this liquid is then replaced with a bisphenol, but even with this procedure, it is very difficult to filter a high-viscosity bisphenol efficiently over an extended period.
As a result of diligent research conducted in light of these problems, the inventors discovered that the foreign matter produced in the reactor in the polycondensation stage is relatively large in diameter, and that microparticles of submicron size are the result of inorganic and organic impurities and other such foreign matter contained in the raw materials, such as the bisphenol or carbonic diester, and also discovered that it is possible to manufacture a high-quality optical polycarbonate with the desired low microparticle content by effectively eliminating this foreign matter. The inventors further turned their attention to the fact that the surface tension of a molten mixture of a bisphenol and a carbonic diester is lower than that of a bisphenol alone, and upon subjecting a molten mixture of a bisphenol and a carbonic diester to high precision filtration with a membrane filter made of a fluororesin, they surprisingly succeeded at eliminating microparticles of submicron size without any pre-treatment, and thereupon arrived at the present invention.